1. Field of the Invention
The present invention relates to an integrated circuit for optical disc.
2. Description of the Related Art
Currently, optical disc apparatuses are widely used that generate RF (Radio Frequency) signals based on light amounts of reflected light of laser beams applied to optical discs (such as CD (Compact Disc) and DVD (Digital Versatile Disc)) to reproduce information recorded on the optical discs in accordance with the RF signals. Therefore, an optical disc apparatus may include a photodetector that receives reflected light of laser beam on a plurality of light-receiving surfaces, and an integrated circuit for optical disc that outputs RF signals based on the reflected light of laser beam received on the plurality of light-receiving surfaces. With reference to FIGS. 5 and 6, the generation of RF signals based on the reflected light of laser beam will hereinafter be described. FIG. 5 is a diagram of light-receiving surfaces A to D that substantially evenly receive the reflected light of laser beam in a photodetector 100, when the laser beam is focused on a track spirally formed in an information recording layer of the optical disc. FIG. 6 is a circuit diagram of a configuration of an integrated circuit for optical disc 101.
The reflected light of laser beam received by the light-receiving surface A of the photodetector 100 is converted into a current in accordance with a light amount of the reflected light by a photodiode 102 of the integrated circuit for optical disc 101. A first operational amplifier 103 converts the current generated by the photodiode 102 into a voltage in combination with a feedback resistor 104 and outputs to a second operational amplifier 105 the output voltage amplified with a gain determined by a resistance value of the feedback resistor 104. The second operational amplifier 105 amplifies the output voltage of the first operational amplifier 103 at a predetermined amplification rate and outputs an output voltage (hereinafter, photoelectric conversion signal A) acquired as a result of the amplification to an adder 107 through a resistor 106A and an external processing circuit of the integrated circuit for optical disc 101. A capacitor 120 shown in FIG. 6 equivalently indicates a parasitic capacitor generated on a signal line for transmitting the photoelectric conversion signal A output from the integrated circuit for optical disc 101 to the external processing circuit through a flexible substrate. The integrated circuit for optical disc 101 has the same configuration (not shown) as the photodiode 102, the first operational amplifier 103, the feedback resistor 104, and the second operational amplifier 105 described above between a Vcc line and a GND line for each of the light-receiving surfaces B to D. As a result, the adder 107 receives input of the photoelectric conversion signals A to D corresponding to light amounts of the reflected light of laser beam received by the light-receiving surfaces A to D, via resistors A to D. The adder 107 outputs an RF signal acquired as a result of adding the photoelectric conversion signals A to D to the inverting input terminal of a third operational amplifier 108. The third operational amplifier 108 amplifies the RF signal applied to the inverting input terminal with a gain determined by a feedback resistor 111 and an input resistor 110 and outputs the signal to the external processing circuit. A capacitor 121 shown in FIG. 6 equivalently indicates a parasitic capacitor generated on a signal line for transmitting a signal corresponding to the RF signal, which signal is output by the integrated circuit for optical disc 101, to the external processing circuit through the flexible substrate. As a result, the RF signal smoothed by the capacitor 121 is input to the output processing circuit through the flexible substrate, and information is reproduced based on the RF signal (see Japanese Patent Application Laid-Open Publication No. 2005-32373).
However, in the above integrated circuit for optical disc 101, the output RF signal may have characteristics shown in FIG. 7B, which is different from ideal characteristics shown in FIG. 7A. This is because, for example, when only the light-receiving surface A of the photodetector 100 is driven to receive the reflected light of laser beam, the photoelectric conversion signals B to D shown in FIG. 8 are generated which have phase speeds faster than that of the photoelectric conversion signal A although only the photoelectric conversion signal A shown in FIG. 8 should normally be generated, and the phase of the photoelectric conversion signals B to D becomes substantially reverse to the phase of the photoelectric conversion signal A in a certain frequency band f (e.g., frequency band around 40 MHz) and thereby the photoelectric conversion signal A is reduced. The photoelectric conversion signals B to D are generated in such a case that a power supply line connected to the photodiode 102 is fluctuated and that the reflected light of laser beam to be received by one light-receiving surface is leaked to other light-receiving surfaces. With reference to FIG. 6, detailed description will hereinafter be made of one cause of the fluctuations of the power supply line, which generate the photoelectric conversion signals B to D when only the light-receiving surface A is driven to receive the reflected light of laser beam.
The Vcc and GND lines connecting the integrated circuit for optical disc 101 and the flexible substrate are configured with wires, etc., and include inductor components corresponding to lengths, etc., of the wires (inductors 122 and 123 of FIG. 6 equivalently indicates the inductor components). Therefore, if the capacitor 121 is charged in accordance with the RF signal, a current ia is supplied from the Vcc line, and the Vcc line is fluctuated when the current ia is supplied to the inductor 122. If the capacitor 121 is discharged in accordance with the RF signal, a current ic is supplied from the capacitor 121 to the GND line, and the GND line is fluctuated when the current ic is supplied to the inductor 123. The fluctuations of the power supply line (the GND line of FIG. 6) connected to the photodiode 102 for the light-receiving surfaces A to D are propagated to the first operational amplifier 103 and the second operational amplifier 105 through parasitic capacitance (hereinafter, parasitic capacitor 109) of the photodiode 102, which is connected in parallel with the photodiode 102, and the photoelectric conversion signals B to D shown in FIG. 8 are generated. The fluctuations of the GND line may be generated not only as described above but also due to external factors such as overlapping of noise components. The level of the photoelectric conversion signal A may be reduced by the photoelectric conversion signals B to D having a reversed phase of the photoelectric conversion signal A in a certain frequency band f, and the RF signal with the waveform shown in FIG. 7B may be generated as described above. If information is reproduced based on this RF signal in the subsequent processing circuit, the information may inaccurately be reproduced or the reproduction of the information may become impossible.